IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) and Session Description Protocol (SDP) to set up and control calls or sessions between user terminals (or user terminals and application servers). Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly.
A User Equipment (UE) can access the IMS by attaching to an access network. If the access network is a Packet Switched (PS) network, such as an Evolved Packet Core (EPC)/Long Term Evolution (LTE) access network, an IMS session can be set up by the UE using SIP signalling. However, many existing access networks only support Circuit Switched (CS) technology for telephony with good enough quality of service, and procedures are well established for dealing with the provision of media and services to a UE accessing the IMS via a CS access network.
There are many occasions when during a call/session it is required to transfer or hand over the call/session from one access network to another. Single Radio Voice Call Continuity (SRVCC) is described in 3GPP TS 23.237 and 3GPP TS 23.216, which specify procedures for handover of a voice call from a PS access to a CS access (e.g. transfer of a VoIP IMS session from an evolved UMTS Radio Access Network—E-UTRAN—to a UTRAN or GSM Edge RAN—GERAN).
Accordingly, Voice over LTE (VoLTE) networks and devices that support co-existence with CS technology will normally have support for SRVCC. 3GPP TS 24.402 specifies procedures for non-3GPP access with the introduction of EPC integrated WLAN. This integrates WLAN as an additionally supported access technology to LTE and CS for a voice service (VoWiFi). However situations can arise where a competing or ‘race’ condition arises between a SRVCC (LTE to CS) handover and a WLAN handover (LTE to WiFi), when a UE leaves LTE coverage.
More particularly, when a user device that supports VoLTE, VoWiFi and CS voice communications as well as SRVCC is attached to a LTE access and has an ongoing call that experiences a drop of signal quality, the device may decide to initiate a voice call handover to WiFi (if available). At the same time, the serving eNodeB (radio access node in LTE) may decide to initiate a SRVCC handover based on measurement reports received from the device. If these competing handover procedures are allowed to continue unchecked a potential call failure may occur.
The embodiments presented herein address these problems, noting that it is normally desirable to maintain PS connectivity in order to maintain communication enrichments such as conversational video.